1. Field of the Invention
The invention is related generally to the field of performing measurements with well logging instruments for the purpose of determining the properties of earth formations. More specifically, the invention provides techniques for making fine measurements of conductivity within a borehole.
2. Description of the Related Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, are diagnostic of the petrophysical properties of the formation and the fluids therein.
In thinly laminated sand-shale sequences or shaly-sand formations, the formation electrical anisotropy becomes important in determining the hydrocarbon saturation from the resistivity measurements. Due to the complexity of subterranean formations determining the electrical properties can be complicated. For example, with typical well logging tools, measurements can be imprecise enough that small indications of reserves are overlooked. In some instances, the small indications can lead to significant or substantial findings. Accordingly, it is prudent to investigate small indications.
Investigation of small indications involves micro-conductivity imaging of the wall of a borehole. Prior research has been conducted in this area, and the need for improvements remains. For example, prior art devices implementing the technology are normally based on galvanic excitation and galvanic reading the signal associated with local conductivity of earth formations. One example is provided in U.S. Pat. No. 6,815,954, entitled “Method and Apparatus for Full Offset Resistivity Imaging for Use in Boreholes,” issued Nov. 9, 2004 to Iwanicki, et al. This patent makes use of micro-resistivity techniques for obtaining an image of a borehole.
Another approach is disclosed in U.S. Patent Application No. US 2005/0242819, entitled “Compact Magnetic Sensor for Multi-Component Induction and Micro-Resistivity Measurements,” published Nov. 3, 2005 and by Gold et al. This application discloses a compact magnetic sensor for use in pad mounted applications and field focusing applications.
Among the various problems with prior art approaches are that galvanic techniques often fail when dealing with a highly conductive formation and low conductive bore mud (oil based mud). Main voltage drop occurs on a low conductive gap (always present) between a button electrode and the formation. The stray electric field and related instability typically causes an unacceptable measurement error as a result.
Other techniques for evaluation of the conductivity of the formation involve probing with an induction-sensing coil that generates an AC magnetic field and corresponding eddy currents in the wall of the borehole. In these embodiments, the induction sensors typically have problems with signal localization and poor sensitivity for variations of the wall conductivity due to poor energy focusing on the spot of interest.
There is a need for advanced methods for providing meaningful results from micro-conductivity evaluations of the walls of a borehole. Preferably, the methods should consistently provide accurate and reliable data and be computationally efficient.